Magnetoelectronics devices, spin electronics devices, and spintronics devices are synonymous terms for devices that use the effects predominantly caused by electron spin. Magnetoelectronics effects are used in numerous information devices, and provide non-volatile, reliable, radiation resistant, and high-density data storage and retrieval. The numerous magnetoelectronics information devices include, but are not limited to, magnetic random access memory (MRAM), magnetic sensors, and read/write heads for disk drives.
Typically, a magnetoelectronic device, such as a magnetic memory element, has a structure that includes multiple ferromagnetic layers separated by at least one non-magnetic layer. In the magnetic memory element, information is stored as directions of magnetization vectors in the magnetic layers. Magnetization vectors in one magnetic layer, for instance, are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer is free to switch between the same and opposite directions that are called “parallel” and “antiparallel”, respectively, and other directions or states. In response to parallel and antiparallel states, and in certain instances other states, the magnetic memory element represents different resistances. The resistance can have minimum and maximum values when the magnetization vectors of the two magnetic layers point in substantially the same and opposite directions, respectively. Accordingly, a detection of change in resistance allows a device, such as an MRAM device, to provide information stored in the magnetic memory element. The difference between the minimum and maximum resistance values divided by the minimum resistance is known as the magnetoresistance ratio (MR).
One type of magnetic memory element, a magnetic tunnel junction (MTJ) element, comprises a fixed ferromagnetic layer that has a magnetization direction fixed with respect to an external magnetic field and a free ferromagnetic layer that has a magnetization direction that is free to rotate with the external magnetic field. The fixed layer and free layer are separated by an insulating tunnel barrier layer that relies upon the phenomenon of spin-polarized electron tunneling through the tunnel barrier layer between the free and fixed ferromagnetic layers. The tunneling phenomenon is electron spin dependent, making the electrical response of the MTJ element or device a function of the relative orientations and spin polarization of the conduction electrons between the free and fixed ferromagnetic layer.
FIG. 1 is a simplified schematic diagram of test circuit 30 for exemplary MTJ device 31. Magnetic tunnel junction device 31 comprises metal-insulator-metal (M-I-M) sandwich 32 and magnetic field source 34. M-I-M sandwich or structure 32 comprises lower electrode 36 of a ferro-magnetic material (e.g., CoFeB), upper electrode 38 of another ferro-magnetic material (e.g., NiFe) separated by very thin dielectric 37 through which tunneling current It can flow in response to voltage Vt applied across electrodes 36, 38. Resistor Ro is provided in series with MTJ 31 so that the electrical response It of M-I-M structure 32 and MTJ 31 to applied voltage Vt can be measured. The ratio Vt/It defines the resistance Rt of M-I-M structure 32 and MTJ 31. In general, It and Rt are non-linear functions of Vt. Arrows 40, 42 indicate the magnetization direction, in electrodes 36, 38, which can be set in particular directions. In an exemplary implementation, the material of electrode 36 is chosen so that magnetization direction 40 is fixed and the material of electrode 38 is chosen so that magnetization direction 42 is free, that is, it can be varied so as to be parallel or anti-parallel or otherwise to magnetization direction 40. This change in magnetization direction 42 is caused, for example and not intended to be limiting, by current Im passing through nearby conductor 34 whose magnetic field 35 intercepts at least electrode 38. By sending a current pulse Im through conductor 34, magnetization direction 42 can be flipped from one orientation to another relative to magnetization direction 40, and will remain in such orientation until a different current pulse is provided. Rt depends upon the relative orientation of magnetization directions 40, 42 and, other things being equal, Rt can have different values depending upon relative orientations of magnetization directions 40, 42. Thus, MTJ device 31 can function as a non-volatile memory or a measuring element whose state is detected by measuring Rt.
Tunnel barrier layer 37 of M-I-M structure 32 is important to the performance of MTJ element 31, as the MR is strongly dependent on the tunnel barrier quality. Furthermore, because future generations of magnetoelectronic devices, such as MRAMs and others, may be scaled to smaller sizes, thinner tunnel barrier layers will be desired. MTJ devices, such as MRAMs, inherently employ very large electric fields across very thin insulating dielectric layers 37. Generally, it is very hard to make such MTJs with uniform, predictable and stable values of resistance Rt. The variations in Rt among nominally identical structures and the variations in the stability of Rt values with time even among substantially identical structures, are significant limitations of MTJ devices, especially when such devices are used in large arrays containing, perhaps, millions of individual MTJ devices. Thus, there is an ongoing need to provide MTJ devices of improved properties.
Accordingly, it is desirable to provide MTJ devices having, among other things, more uniform, predictable and stable properties. In addition, it is desirable that the methods, materials and structures employed be compatible with present day manufacturing capabilities and materials and not require substantial modifications of manufacturing procedures or substantially increase manufacturing costs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.